Method of manufacturing an electrode

Abstract: The subject invention relates to electrode structures that are adaptable for primary and electrically rechargeable electrochemical wafer cells. A flat wafer cell is disclosed that includes conductive, carbon-filled polymeric outer layers that serve as electrode contacts and as a means of containment of the cell. Multi-cell, higher voltage batteries may be constructed by stacking individual cells. Specially formulated electrodes and processing techniques that are compatible with the wafer cell construction are disclosed for a nickel-metal hydride battery system. This invention is further directed to fabrication of electrodes from electrochemically active material prepared by coating non- to low-conductive particles of a metal hydroxide or metal oxide powder with nickel using a electroless nickel coating process. The cell design and electrode formulations provide for individual operation of a vented or low pressure sealed cell and/or for operation of these cells in a stacked array in an outer battery housing.

Description: FIELD OF THE INVENTION The field of present invention, relates to a method of making an electrode. BACKGROUND OF THE INVENTION Multi-cell batteries that are constructed in a broad range of electrochemical systems are typically packaged in cylindrical or prismatic housings. Individual cells are connected in series by conductive links to make the multi-cell batteries. Such construction approaches provide for good sealing of the individual cell compartments and for reliable operation. However, such constructions allocate a large fraction of the multi-cell battery's weight and volume to the packaging and, thus, do not make full use of the potential energy storage capacity of the active components of the cell. For improving battery energy storage capacity on a weight and volume basis, packaging approaches are sought that reduce packaging weight and volume and that provide stable battery performance and low internal resistance. These objectives have led to the pursuit of a bipolar construction in which an electrically conductive bipolar layer serves as the electrical interconnection between adjacent cells as well as a partition between the cells. In order for the bipolar construction to be successfully utilized, the bipolar layer must be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good electrical contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell. These requirements are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the bipolar layer and in alkaline batteries due to the creep nature of the electrolyte. Achieving the proper combination of these characteristics has proved very difficult. For maintenance-free operation it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that are structurally poor for containment of the gases present or generated during cell operation. In a sealed cell construction, gases are generated during charging that need to be chemically recombined within the cell for stable operation. To minimize weight of the structures used to provide the gas pressure containment, the battery should operate at relatively low pressure. The pressure-containment requirement creates additional challenges on designing a stable bipolar configuration. Despite a number of patents and considerable effort at making a bipolar construction for the lead-acid and nickel-cadmium systems such batteries are not commercially available, (U.S. Patent No. 4,098,967). Construction of a flat metal hydride battery is even more difficult because many of the metal hydride alloys used to make metal hydride batteries operate at elevated hydrogen pressures. The bipolar construction has been successfully employed in the flat plate construction of the Leclanche MnO 2 -Zn system as a primary battery, U.S. Patent No. 4,098,965. Since a primary battery is non-rechargeable, the materials-stability problem is less severe and the aqueous chloride electrolyte may be contained without unreasonable difficulty. Another problem of prior art electrochemical cells relates to the material problems encountered with metal hydride electrodes. Electrochemically reversible metal hydride electrodes operate by the absorption of hydrogen in the lattice of the metal hydride alloy during electrochemical charging of the cell. A number of alloy formulations have been identified of the so-called AB 5 and AB 2 structure that can function in this manner, for example, as disclosed in U.S. Patent Nos. 4,488,817 and 4,728,586. To insure reasonable rates of reaction and transport of hydrogen, such electrodes may be prepared from alloy powders typically having an average particle size of about 50 microns. Fabricating an electrode structure from the alloy powders may be accomplished by sintering the metal powders or by using polymeric binders. However, conventional techniques do not yield electrodes that make good and stable contact to the cell face of the conductive outer layer in a bipolar construction. Metal hydride alloys can fragment during repeated cycling as the alloy undergoes expansion and contraction each time the hydrogen enters and leaves the lattice. It is also recognized that oxygen and or the electrolyte can react with the hydride alloy and cause deterioration of the hydrogen storage capacity of the hydride alloy. The present invention provides a method for achieving the packaging benefits of bipolar construction for rechargeable multi-cell batteries and of overcoming the materials and construction problems of previous approaches. Although the materials of construction for each type of cell are specific to each battery chemistry, the generic bipolar construction disclosed herein may be used for many types of electrochemical cells. In particular, the descriptions and approaches that follow relate specifically to the rechargeable nickel-metal hydride chemistry but may generally be adaptable to other chemistries. The electrically rechargeable nickel battery electrode that may be used in an electrochemical battery has proven itself to have good cycle life and discharge rate capability in a number of battery systems including nickel/cadmium, nickel/iron, nickel/zinc, nickel/hydrogen and nickel/metal-hydride. The generally accepted reaction at the nickel electrode is as follows: NiOOH + H 2 O + e - → Ni(OH) 2 + OH - The nickel active material cycles between the two oxidation states (+2 and +3) during the charge/discharge process. Based on a one-electron change, 3.43 grams of Ni(OH) 2 is theoretically reacted per ampere hour of capacity. The excellent cycle life of nickel electrodes can be attributed to the low solubility of the charge/discharge species in the alkaline electrolyte employed and the fact that the active material does not chemically or physically change significantly during cycling. In order for the active material to function in a battery electrode, it must be in electrical contact with an electrode current collector for electron flow and in physical contact with the electrolyte for the electrolytic ion reaction. The nickel active material is a relatively poor electronic conductor in its two oxidation states, is not cohesive and goes through a modest expansion and contraction between charge and discharge. To overcome these shortcomings practical battery electrodes are fabricated in such a way that the active material is structurally contained to avoid sheathing and held in intimate contact with an electrically conductive component to ensure the flow of electrons from the active material. This has been achieved with pocket electrodes in which a mix of active material and nickel flake or graphite is packed into perforated tubes or pockets, developed by Edison, United States Patent No. 723,450 (1902), and Jungner, Sweden Patent No. 11,132 (1889); sintered electrodes in which active material is chemically or electrochemically impregnated into porous sintered nickel powder, German Patent No. 491,498 (1928); plastic bonded electrodes in which a mix of graphite and active material is bonded together with a Teflon® binder, United States Patent No. 3,898,099, and pasted electrodes in which nickel hydroxide active material and a binder is pasted into a porous nickel foam or fiber plaque, United States Patent No. 4,844,999. In the above electrode types, the required combination of properties, specifically including structural containment of the active material, access to electrolyte and electronic contact to the active material is achieved. The relative merits of the different electrode types are evaluated on the basis of capacity per unit volume, capacity per unit weight, cost per unit of capacity, electrode current drain rate capability, cycle life, capacity retention, temperature characteristics, charge efficiency, operating voltage and process hazardous waste. The characteristics of each electrode type are a function of the weight and volume ratios of active material to additives and support structure, percent utilization of the active material, conductivity of the composite structure and stability of the electrode structure. A given application may place greater emphasis on a particular electrode characteristic since no existing electrode type is superior in all characteristics. Therefore, different electrode types are typically selected for different applications. The limitation of the prior art is that finished electrodes typically contain 30-60% added inert weight, in addition to the weight of the active material and, furthermore, require costly structures and processes. Finished electrodes fall far short of the theoretical energy capabilities of the active material by itself. The same issues of electrode characteristics, costs and limitation exist with other battery electrode chemistries and the invention described herein is also applicable to other electrode chemistries. ADVANTAGES AND SUMMARY OF THE INVENTION The invention is defined in the accompanying claims. The invention provides a method of making an electrode comprising: dry mixing a combination of an electrochemically active material and polytetrafluoroethylene particles in a blending mill to form a mixture; dry rolling the mixture into a thin layer; dry kneading the thin layer by sequentially folding and rolling the thin layer such that the polytetrafluoroethylene particles form an interconnected and fibrous network in which the electrochemically active material is embedded; calendering the folded and rolled thin layer to form a substantially flat porous sheet; and cutting a substantially flat electrode from the porous sheet. Methods for preparing a battery electrode can also include coating particles of an electrochemically active material with a sensitizing agent to produce particles having a sensitized surface, reacting a nucleating agent with the sensitized surface to produce particles having a surface activated for electroless deposition, immersing the particles having an activated surface in an electroless deposition bath, depositing a microporous metal layer on the activated surface to produce metal-coated particles of the electrochemically active material and removing the metal-coated particles from the electroless bath. The metal-coated particles can then be used as the electrochemically active material for making an electrode as described above. Electrodes made according to the methods described above can be used in a bipolar batter construction for rechargeable multi-cell batteries that overcomes the above-cited problems of prior art bipolar constructions. Bipolar designs that have improved energy storage capacity while still providing stable and efficient battery performance can therefore be provided. Electrodes made according to the methods described above can be used in bipolar battery constructions using flat electrochemical cells having a sealed configuration. The methods described above can be used to provide bipolar battery constructions wherein metal hydride electrodes are used. The methods described above can be used to provide improved metal hydride electrode structures. The methods described above can be used to prepare flat metal hydride structures that may be used in rechargeable bipolar battery constructions. While embodiments of the invention are illustrated for improved active materials having use in rechargeable nickel electrodes, the techniques may be used for other battery active materials that would benefit from enhanced electronic conductivity, for example, a battery active material of manganese dioxide, cobalt oxide, vanadium pentoxide, etc. Another object of the present invention is to provide a method of fabricating plastic-bonded electrodes using active material having enhanced electronic conductivity. The desired result is an electrode of comparable or improved performance to electrodes which use sintered or foam plaques that are expensive as compared to electrodes manufactured using the subject low-cost plastic bonded process. The desired result is achieved by coating the active material with an ultra-thin conductive skin which enhances the electronic conductivity of the finished electrode. An unexpected result of the present invention is that specially controlled electroless nickel coating of particles of nickel hydroxide produce an active material containing 5-30 weight percent nickel. Particles coated using this process can provide excellent particle-to-particle contact when bonded and pressed into an electrode. Moreover, such electrodes do not inhibit ion transport to the active material, as is required for effective electrochemical activity. The active material coated using this process may be fabricated into electrodes by a number of processes including pocket electrodes or pasting into foam or porous fiber plaques or by plastic bonded techniques. According to embodiments of the invention, the enhanced-conductivity active material is structurally bonded by a unique dry Teflon® bonded process to form the electrode structure. The methods described above can also be used to produce nickel electrodes providing cost and performance improvements over prior art nickel electrode. Electrodes made according to the methods described above can be used to make a bipolar electrochemical battery comprising: a stack of at least two electrochemical cells electrically arranged in series with the positive face of each cell contacting the negative face of the adjacent cell, wherein each of the cells contains (a) a metal hydride electrode; (b) a nickel electrode; (c) a porous separator between the electrodes, wherein the separator contains an electrolyte; (d) a first electrically conductive outer layer in electrical contact with the outer face of the metal hydride electrode; and (e) a second electrically conductive outer layer in electrical contact with the outer face of the nickel electrode; wherein the outer layers are sealed peripherally to an electrically non-conductive polymeric material such as to form a sealed enclosure containing the electrodes, the separator and the electrolyte. Further objects and advantages of the subject invention will be apparent to those skilled in the art from the following detailed description of the disclosed bipolar electrochemical battery and the methods for producing bipolar electrochemical batteries, and to the metal hydride electrodes used therein. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an overview of a wafer cell. Fig. 2A shows a three-dimensional view of a multi-cell stack of wafer cells and Fig. 2B shows a two-dimensional side view of a multi-cell stack of wafer cells. Fig. 3 shows a side view of a sealed battery housing. Fig. 4 shows a side view of a sealed battery housing containing a hydraulic fluid. Fig. 5 shows a side view of a multi-electrode configuration in a single wafer cell. Fig. 6 shows a wafer cell having a back side oxygen recombination arrangement. Fig. 7A shows the strip chart recording of the cell voltage of cell #113 for cycle numbers 104, 112, 242, 250, 368 and 376. Fig 7B shows the strip chart recording of the cell voltage of cell #113 for cycle numbers 480, 488, 608, 616, 680 and 688. Fig. 8 shows the strip chart recording of the cell voltage of cell #121 for cycle numbers 4, 5, 128, 136, 288 and 296. Fig. 9 shows the strip chart recording of the cell voltage of cell #144 for cycle numbers 101, 102, 103, 942, 943, 944, 1509, 1510 and 1511. Fig. 10 shows the strip chart recording of the cell voltage of cell #134 for cycle numbers 168, 176, 296, 304, 408 and 416. Fig 11 shows the strip chart recording of the cell voltage of cell #170 for cycle numbers 98, 99, 100, 298, 299, 300, 398, 399 and 400. Fig. 12 shows the strip chart recording of the voltage of a stack of five cells for cycle numbers 2 and 5. Fig. 13 shows a scanning electron microscope (SEM) photograph of uncoated electrochemically active material. Fig. 14 shows a SEM photograph of uniformly coated electrochemically active material. Fig. 15 shows the strip chart recording of the voltage at periodic cycles for cell #296. Fig. 16 shows the strip chart recording of the voltage at periodic cycles for cell #342. Fig. 17 shows the strip chart recording of the voltage at periodic cycles for cell #A300. Fig. 18 shows the strip chart recording of the voltage at periodic cycles for cell #305. Fig. 19A shows the strip chart recording of the voltage at periodic cycles for cell #393. Fig. 19B shows the minimum voltage as a function of the cycle number for cell #393. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS While the following description of preferred embodiments of the present invention is intended to provide detailed instructions that would enable one of ordinary skill in the art to practice the invention, the scope of the invention is not to be limited to the scope of the specific product or process details hereinafter provided. The bipolar electrochemical battery of the subject invention first involves preparing a single electrochemical cell. Fig. 1 shows a schematically illustrative embodiment of a wafer cell 1 comprised of a pair of electrodes contained between two conductive, carbon-filled outer layers 2 and 3 that make electrical contact to the positive and negative electrodes, 4 and 5 , respectively. The electrodes are prevented from coming into direct physical electrical contact by a separator 6 that typically is porous so as to contain an electrolyte. The electrolyte typically comprises an aqueous solution of one or more alkali hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide. The separator material typically comprises synthetic resin fibers such as polyamide or polypropylene fibers. In order to enhance the electrical contact, a conductive paste or cement may be used between each of the carbon-filled outer layers and the respective electrode with which it is in contact. In the preferred embodiment of the present invention, the negative electrode 5 is a bonded metal hydride alloy powder that can electrochemically and reversibly store hydrogen. Such alloys may include materials disclosed in U.S. Patent Nos. 4,487,817 and 4,728,586 but the subject invention is not limited to the formulation of materials disclosed therein. These alloy formulations may include what are commonly referred to as Mischmetal hydride alloys, which may be comprised of an alloy of hydride-forming metals such as MnNi 3.5 Co 0.7 Al 0.83 . The positive electrode 4 of the present invention is preferably what is typically referred to as a nickel-type electrode, or more simply, as a nickel electrode. Nickel hydroxide is the active component of a nickel electrode. Prior art nickel electrodes are illustrated in German Patent No.491,498 and British Patent No. 917,291. As indicated in Fig. 1, the preferred construction of the present invention comprises an electrochemical cell wherein the electrodes, the separator between the electrodes and the two outer layers are each substantially flat and in tight physical and electrical contact with the adjacent component. The design illustrated in Fig. 1 permits construction of a thin cell that is referred to herein as a wafer cell. In order for the electrodes, the separator between the electrodes and the electrolyte to be contained within an enclosed wafer cell, the flat outer layers 2 and 3 have a larger physical area than the electrodes, such that each outer layer extends beyond the electrode around the entire perimeter of the adjacent electrode. A non-conductive material 7 may be sealed peripherally to the outer layers to form a border material around the entire perimeter of the electrodes such as to form a sealed enclosure containing the pair of electrodes, the separator and the electrolyte within the wafer cell. The border material is preferably of a polymeric material that may be heat sealed to the outer layers. The polymeric material of the subject invention is preferably a vinyl polymer. The enclosed wafer cell may be completely sealed or it may be provided with vents for release of excess pressure that may build up in the cell during charging. Since the flat cell construction is a poor physical configuration for a pressure-containment vessel, the use of hydride alloys that operate at atmospheric pressure are preferred. If a completely sealed configuration is used, a design that is electrochemically limited by the capacity of the positive electrode is preferred. For this type of design, oxygen gas is generated at the end of the charging cycle at the positive electrode before the total available hydrogen storage capacity of the hydride electrode is fully utilized. Oxygen produced at the positive electrode may migrate to the negative hydride electrode and chemically recombine with the hydrogen in the hydride electrode so as to help prevent excessive build-up of pressure. The chemical recombination of oxygen and hydrogen is referred to herein as the oxygen recombination reaction. The present invention further relates, as described in more detail hereinafter, to providing means for enhancing the migration of oxygen gas to the negative electrode and for promoting efficient chemical recombination of the oxygen with hydrogen at the hydride electrode surface. In addition to helping prevent excessive hydrogen build-up, the efficient migration and removal of the oxygen by chemical recombination with hydrogen also helps reduce the tendency for deterioration of the negative hydride electrode by oxidation after many charging cycles. The separator between the electrodes typically has a porous structure for absorbing and containing the electrolyte within the cell. In a preferred embodiment of the present invention the separator is comprised of two layers of non-woven nylon and the electrolyte is comprised of an alkaline solution. Preferably the alkaline electrolyte is a mixed hydroxide of potassium and lithium hydroxide. The separator extends beyond the edge of the electrodes so as to prevent direct electrical contact between the electrodes. The individual electrodes each may be connected to current collectors for carrying current between the adjacent cells. Preferably, the current collectors are not necessary since the current path between adjacent electrodes is relatively short and the area of physical and electrical contact between the adjacent cells is large relative to the total area of the adjacent components. In addition, the electrodes are typically conductive enough for cell operation without having current collectors that add weight and complexity to the cell. Fig. 2A and Fig. 2B show a multi-cell battery 8 made by stacking several wafer cells 1 . The wafer cells are electrically arranged in series with the positive face of each cell contacting the negative face of the adjacent cell. The end cells have metal foil contacts, 9 and 10 , respectively, to conduct the electric current from the battery stack to the battery terminals. The cell-to-cell contact or the contact between the end cells and the metal foil contacts may be enhanced by use of a conductive paste or cement. The compact stack assembly is held in compression to insure uniform physical contact between the adjacent cells and between the respective layers within each cell. The stack compression can be achieved by means of ridged end plates 11 and 12 having external tie rods 13 and 14 wrapped around the perimeter of the stack, as shown in Fig. 2B, or by having internal tie rods 16 , as shown in Fig. 3, that penetrate through sealed holes provided in the individual electrochemical cells. The holes are sealed such as to prevent electrical contact between the tie rods and the electrically conductive components of the cell. Alternatively, the stack may be contained in an outer battery housing that serves as the battery housing 8 . To allow for electrode expansion and irregularities in the stack, the stack may be held in compression by means of a layer of sponge rubber 15 between one or both of the metal foil contacts 9 and 10 and the end plates, 11 and 12 , respectively, of the outer housing. Fig. 2B is shown with only one layer of sponge rubber, 15 . Alternatively, a spring or a gas-filled compressible pad may be used instead of the sponge rubber. If the cell stack is contained in an enclosed outer housing, the outer housing can serve to provide stack compression and the housing may be sealed or vented. Fig. 3 shows an embodiment of the invention in which multiple cells each have small vent ports 17 and the cells are contained in a sealed container which serves as the battery housing 8 . The battery housing can be provided with a pressure measuring device. Such a device may be a pressure gauge, a transducer and/or a pressure switch 19 . The pressure measuring device may be used for monitoring the battery pressure and for regulating the magnitude and duration of the charging current during the charge cycle. Such regulation of the charging current is herein referred to as charge control. The stack may contain internal tie rods 16 to insure uniform compression and contact over the entire plane of the cells. The sealed container may have a pressure relief valve 18 to vent internal gases. The individual wafer cells 1 may be made according to the methods disclosed herein and the remaining components shown in Fig. 3 may be made using known methods or obtained from supply sources known to one skilled in the art. Fig. 4 shows a similar arrangement as above in which the individual cells are sealed in an outer housing 8 and the void space around the cells is filled with an electrically non-conductive fluid 20. The fluid may be one of many different hydraulic fluids that are known and available. The outer housing contains a pressure switch and/or a pressure indicator 21 . If gas pressure is generated within any cell, the cell envelope expands slightly and exerts pressure on the fluid. Since the fluid is non-compressible, the pressure is transmitted to the pressure switch which can be used for charge control. This technique provides for individual sealed cell operation and allows for the first cell that generates pressure to control the battery charge. This avoids reaching an excessive pressure in any cell and eliminates formation of a gas or a liquid interface between individual cells during operation. Fig. 5 shows an embodiment of a single wafer cell that contains multiple electrodes to increase cell capacity and/or electrode area. In this case the positive and negative electrodes, 4 and 5 , respectively, contain current collectors, 22 and 23 , respectively, which are folded to provide electrical contact with the entire surface of each electrode. Fig. 6 shows an embodiment of a single wafer cell having an improved means for chemically recombining the oxygen gas produced during sealed cell operation. In this case, there is a porous spacer 24 between the back side of the hydride electrode and the conductive, carbon-filled outer layer. The spacer is electrically conductive so as to provide electrical contact of the electrode with the cell face. Preferably, the spacer is porous graphite paper. This back side recombination configuration provides a path 25 for migration of oxygen to the back side of the hydride electrode where it can recombine with the hydrogen. The back side of the hydride electrode can be wet proofed and/or provided with a catalyst to enhance oxygen recombination. Wet proofing may be carried out using known methods for increasing the hydrophobic nature of the surface. The catalysts may be selected from those known to enhance the oxygen recombination reaction. The porous spacer can also be wet proofed and/or provided with a catalyst to enhance the recombination reaction. These approaches can increase the rate of cell overcharge and keep the cell pressure at acceptable levels during overcharge. The back side recombination configuration allows for use of dense separator materials and an increased quantity of electrolyte in the separator since the oxygen generated during overcharge has an alternative path to reach the negative electrode and thereby prevent excessive oxygen build-up. Another important aspect of the subject invention relates to a method for the fabrication of electrodes and to techniques for insuring good electrical contact of the active electrodes with the conductive cell faces of the wafer cell and with the electrolyte. In particular, an electrode fabrication method is herein disclosed in which an electrochemically active material is held together with a fibrous lace-like network of a polymeric binder. The term "electrochemically active material" is a term that would be readily understood by one of ordinary skill in the art. In particular, the term relates to materials that may be used as the active component in an electrode of an electrochemical cell. Such materials typically comprise a metal or a metal-containing material that readily participates in an electrochemical reaction. For example, for alkaline storage batteries, the electrochemically active material used in the positive electrode may be made from the oxides or hydroxides of nickel, silver, manganese, copper or mercury, and from oxygen. The electrochemically active material used in the negative electrode of an alkaline storage battery may be fabricated from various geometric forms of cadmium, iron or zinc and from hydrogen. Preferably, the electrochemically active material of the negative electrode of the subject invention comprises a metal hydride alloy powder. Metal hydride alloy powders are well known in the art and such powders are readily available from known commercial suppliers. Preferably, the metal hydride powders comprise particles having an average particle size from about 20 to about 50 microns. Most preferably, the metal hydride particles have an average particle size of about 50 microns. As an example of a preferred embodiment of the subject method, a combination of a metal hydride alloy powder and a powder of the polytetrafluoroethylene polymer known as Teflon® is first dry mixed in a high speed blending mill. The mixture is rolled into a thin layer and the layer is then dry kneaded by sequentially rolling and folding in a shearing action that causes the discrete Teflon® binder particles to fibrillate into a fibrous, lace-like network that has a cohesive structure. The subject invention is directed to the discovery that by sequentially folding and rolling a layer including Teflon® particles, wherein the direction of the folding and rolling is turned about 90 degrees from the direction of the immediately preceding step, a continuously interconnected, fibrous and lace-like structure can be produced in a manner such as to form a cohesive structure in which the electrochemically active particles are embedded. The subject invention is specifically directed to the discovery that Teflon® particles are particularly suitable for use in forming this fibrous, lace-like structure. After the mixture is sufficiently worked by the folding and rolling steps, it is then calendered to the desired electrode thickness. The subject method differs from that described in U.S. Patent No. 3,898,099 in that the subject method uses Teflon® having a finer particle size and, furthermore, the subject method is a dry method that is accomplished without a lubricating fluid. In particular, the particle size of the Teflon® powder is preferably less than about 20 microns. Using this dry method it is possible to prepare cohesive porous sheets of a metal hydride powder with unexpectedly small quantities of the Teflon® binder in the range, for example, of about 0.5 to about 5 weight percent. This quantity of binder is sufficient to prepare battery electrodes that do not disintegrate during operation. A significant feature of the subject method is that small quantities of binder may be used without the binder completely overcoating the electrochemically active material as happens with other solvent-binder methods. The subject method results in achieving the two competing objectives of providing particles that have good inter-particle contact and of producing particles that are capable of being uniformly wettable with the electrolyte. The subject dry-processing method eliminates the hazard of a working fluid and, thus, eliminates the need to remove the working fluid from the electrode strip before assembly of the electrode in the cell. Another feature of the dry process is ease of adaptability to automated production and it does not use any processing chemicals that have to be discarded. Another feature of the dry process is ease of adaptability to automated production and, in addition, the disclosed process does not use processing chemicals that have to be discarded. Even though this method yields a bonded porous sheet of good structural integrity, metal hydride electrodes fabricated in this manner still perform very poorly due to poor particle-to-particle electric contact as a result of an oxide film that is typically present on hydride powders. It was discovered that incorporation of additive particles such as cupric oxide powder into the mixture can overcome this problem. Although the theory of how the additive particles of cupric oxide improve the cell performance has not been confirmed or proven, it is believed that the improvement is provided in the manner as described below. The scope of the subject invention is, however, not to be limited to the theory hereinafter described. Cupric oxide is slightly soluble in the alkaline electrolyte used in the cell. By allowing the electrode containing the cupric oxide particles to soak in the electrolyte prior to the first charge cycle, the cupric oxide may enter into solution. During the first charge cycle of the cell the cupric oxide in solution and in the pores of the electrode may be electrochemically reduced and converted to metallic copper. It is believed that the metallic copper deposits on the metal hydride particles throughout the electrode structure as well as on the interface between the electrode layer and the conductive outer layer, such that an interconnected metallic copper layer is produced throughout the electrode structure. If properly and uniformly deposited, the interconnected metallic copper layer would not be expected to retard the metal-hydrogen reaction at the hydride surface, but could create a conductive network that would improve the overall electrical integrity of the electrode structure and, thus, its electronic conductivity. The interconnected metallic copper layer produced in this manner may be formed uniformly throughout the porous electrode structure, but the metallic layer is preferably present only to the extent required to give adequate electrical contact throughout the electrode. The interconnected metallic-copper layer may also serve as a protective layer to reduce alloy oxidation as well as to enhance the oxygen recombination reaction for sealed cell operation. By enhancing the chemical recombination of hydrogen and oxygen, the incorporation of cupric oxide particles in the electrode, which are converted into an interconnected metallic copper layer, may also assist in balancing the charge of the nickel and the metal hydride electrode for sealed cell operation. Furthermore, the deposited metallic copper layer may also serve as a lower voltage reserve capacity in the metal hydride electrode, if the electrode is discharged completely, thereby avoiding reversal of the metal hydride electrode. Although the subject method is described in terms of using cupric oxide particles as the additive particles that are believed to be electrochemically converted into an interconnected metallic layer, it is to be understood that other additive particles comprising a material that is capable of being electrochemically converted into a metal layer also fall within the scope of the present invention. The term "additive particles" is herein defined to refer to such particles that are capable of enhancing the electrical contact in the manner described. Another aspect of the present invention relates to a preparation technique for formulation of the nickel electrode. Nickel electrodes for rechargeable nickel alkaline batteries may be of the sintered, pasted or plastic-bonded type. However, it is necessary that the nickel electrode makes effective and stable contact with the conductive polymeric cell face of the wafer cell. One approach employed herein is, by the dry technique described above, to fabricate a Teflon®-bonded layer from a mixture of nickel hydroxide and graphite and cobalt monoxide and of the Teflon® powder. During the initial electrolyte-soak period a portion of the cobalt monoxide enters into solution and redeposits as a conductive layer at the electrode-conductive polymeric interface. This mechanism is described in U.S. Patent No. 4,844,999. The nickel electrode may also be prepared by pasting a mixture of carboximethylcellulose binder ("CMC"), nickel hydroxide and cobalt monoxide in a nickel foam. This pasted, foam-nickel electrode is capable of making good electrical contact to the conductive polymeric face of the wafer cell. In still another of the preferred embodiments of the subject invention, the nickel electrodes are fabricated from electrochemically active material prepared by coating non- to low-conductive particles of a metal hydroxide or metal oxide powder with nickel using a electroless nickel coating process. The metal hydroxide powder or metal oxide may be selected from those materials capable of functioning as an electrochemically active material, but which require enhanced conductivity for satisfactory operation. Such materials include MnO 2 , CoO 3 , VO 3 , and Ni (OH) 2 and may be readily obtained from commercially available sources. Preferably, the metal hydroxide or metal oxide powder is comprised predominantly of substantially spherical particles, although it is to be understood that powders having other particle shapes may also be used. The particles preferably have an average particle size from 5-15µ. Most preferably, the metal hydroxide or metal oxide powder is comprised of spherical particles of Ni(OH) 2 . The subject electroless nickel coating process may employ various materials and steps known in the art for producing an electroless metal coating. Such processes may include pre-treating the substrate particles with a sensitizing agent and then with a nucleating agent prior to plating the pre-treated substrate particles in an electroless bath with a thin nickel coating or film. The sensitizing agent is understood to sensitize the surface of the particles of electrochemically active material in such a manner so as to promote deposition of the nucleating agent on the surface of the particles in the subsequent step. The nucleating agent is understood to activate the surface of the particles by providing nucleating sites for catalytically depositing a nickel layer on the activated surface from the electroless deposition bath. The overall process comprises the electroless deposition of nickel by employing bath compositions and conditions such that a substantially uniform micro-porous deposit is believed to be coated on the entire surface of substantially all the electrochemically active particles. It was surprising that such electroless processes could be used to plate particles of an electrochemically active material with a nickel coating such that the coated particles would have excellent particle-to-particle contact when bonded and pressed into an electrode and wherein, in addition, the plated coating surrounding the active material particles does not inhibit ion transport to the active material. The coating, thus, provides improved electronic conductivity to the electrochemically active material while not inhibiting electrochemical activity when an electrode prepared from such coated particles is used as part of an electrochemical cell. While the preferred embodiments of the subject electroless metal coating process are directed to providing nickel coatings on electrochemically active materials, it is to be understood that still other metals and electrochemically active materials that are capable of being used as electrode materials in an electrochemical cell are within the scope of the subject invention. In a preferred embodiment of the subject invention, nickel hydroxide powder is initially soaked in a sensitizer solution, for example, a solution containing a sensitizing agent such as SnCl 2 and HCl. Still other sensitizer solutions may be used depending on the substrate particles to be plated. After soaking in the sensitizer solution, the nickel hydroxide may be removed by filtering so as to obtain a wet nickel hydroxide powder. The wet powder may then be soaked in a nucleator solution, for example, a solution of a nucleating agent such as PdCl 2 and HCl. Still other nucleator solutions may be used dependent on the substrate particles to be coated and on the metal coating to be plated. After soaking in the nucleator solution, the wet nickel hydroxide powder may once again be removed and recovered by filtering. The wet powder may then be placed in an electroless deposition bath. While many different electroless deposition bath solutions may be used to prepare the coated particles of active material, in a preferred embodiment of the subject invention, the wet nickel hydroxide powder is placed in an aqueous solution prepared from hydrated nickel sulfate (NiSO 4 ·6H 2 O), sodium hypophosphite (NaH 2 PO 2 ), and sodium pyrophosphate (Na 4 P 2 O 7 ), with the solution adjusted to a Ph of about 10 with NH 4 OH. In another of the preferred embodiments of the subject invention, the nickel sulfate may be provided as part of a composition including cobalt sulfate, for example, 10% CoSO 4 and 90% NiSO 4 , which is provided in addition to the sodium hypophosphite and sodium pyrophosphate. The solution may then be mildly heated to a temperature of about 35°C. During this process, the nickel hydroxide particles become plated with a thin nickel coating. The thickness of the coating may be controlled by varying the concentration of the active ingredients in the electroless bath composition. After the plating ceases, which is indicated by a cessation of gassing, the wet powder, which is now comprised of nickel-coated particles, may then be removed by filtering and washed with water to neutrality. The wet powder may be vacuum dried. The weight gain produced by the metal coating may be from about 5 to about 35 weight percent. Preferably, the weight gain produced by the metal coating is about 15 to about 25 weight percent. Although a powder of the starting nickel hydroxide active material is virtually a non-conductor, the coated active material powder when held in compression at 14 psi may have a bulk resistivity of less than 2 ohm-cm, preferably, 0.4 to 1.0 ohm-cm. It was surprisingly found that such low bulk resistivity values could be achieved without significantly blocking the electrochemical activity of the active material. In particular, it was found that the electrochemical Faradaic utilization efficiency of the active material could be as high as 75 percent or more of the theoretical maximum value. A Faradaic utilization efficiency of about 90 percent or higher is even disclosed herein. An electrode may be made according to the subject invention wherein the weight of the electrochemically active material, not including the weight of the metal coating thereon, may comprise greater than 55% of the total electrode weight. The capacity per unit volume of electrodes made according to the subject invention may be 5.5-6.5 ampere-hours per 16.4 cm 3 (cubic inch) or greater. The nickel-coated particles may then be fabricated into an electrode using known formulation methods for example, by preparing a paste including the nickel-coated particles. Alternatively, electrodes may be made with or without current collectors by using the dry-mixing, folding and rolling technique disclosed herein. Mono-polar electrode structures may be fabricated by laminating a metal current collector with tab between a pair of porous electrode strips. Preferably, the nickel-coated active material is fabricated into an electrode using the dry technique disclosed. The electrodes prepared from the nickel-coated active material may then be fabricated into an electrochemical cell using known fabrication methods. Preferably, the nickel-coated particles are fabricated into an electrochemical cell using the methods disclosed herein. This invention will now be described in detail with respect to the specific preferred embodiments thereof, it being understood that the materials and the process steps of the following examples are intended to be illustrative only. The invention is not intended to be limited to the materials, conditions, process parameters and the like recited herein. EXAMPLES OF THE INVENTION Example 1 A single cell was fabricated which consisted of one positive nickel electrode and one negative metal hydride electrode assembled in an arrangement as shown in Fig. 1. The hydride electrode was prepared by blending a mixture consisting of 45 grams of a Mischmetal hydride alloy, 0.5 grams of Teflon® powder and 4.5 grams of CuO. The Mischmetal hydride alloy used herein was comprised of an alloy of Mn Ni 3.5 Co 0.7 Al 0.8 · The hydride alloy, received as about 0.32-0.64 cm (1/8 to 1/4 inch) particles, was fragmented by dry pressure hydrating five times between vacuum and 13.8 bar (200 psi) to produce an average particle size of about 50 microns. The mixture was blended in a high speed blender for two 30-second periods. The mixture was then rolled out to a layer approximate 1.52 mm (0.060 inch) thick, and then folded and rolled to 1.52 mm (0.060 inch) thickness in a direction about 90 degrees from the original direction. The above folding and rolling in the rotated direction was sequentially repeated seven times to a point wherein the Teflon® powder was fibrillated to form a fibrous, lace-like network which contained and bonded the other ingredients. For each step, the folding and rolling was carried out in a direction about 90 degrees from the folding and rolling direction of the immediately preceding step. The strip was then calendered to a final thickness of 0.51 mm (0.020 inches). A 7.62 x 7.62 cm (3x3 inch) electrode, weighing 11 grams, was cut from the strip and assembled in the cell. The nickel electrode was prepared using a method similar to that described for the hydride electrode. The mixture contained 1 gram of Teflon® powder, 1.5 grams of cobalt monoxide, 15 grams of graphite powder and 32.5 grams of nickel hydroxide powder. The final strip was calendered to a thickness of about 1.01 cm (0.040 inches). A 3x3 inch electrode weighing 10 grams was cut from the strip. The electrode was then pressed at about 138 bar (2,000 psi) in a hydraulic press to a thickness of about 0.84 mm (0.033 inches) prior to assembly in the cell. Two layers of a non-woven nylon separator were placed between the electrodes. The outer layers of the cell were constructed from a conductive, carbon-filled polymeric film manufactured by Graphics Inc. A frame border of non-conductive vinyl polymer was sandwiched between the inner faces of the outer layers and heat sealed on three sides. Nickel foil layers with a thickness of 0.051 mm (0.002 inches) were placed on the outer faces of the outer layers. The cell assembly was then placed between two rigid acrylic plates which contained peripheral bolts to hold the assembly together. The cell was filled with 30% KOH-1% LiOH electrolyte, allowed to soak for 24 hours and then subjected to three formation cycles. A formation cycle is comprised of 15 hours charge at 150 Ma and discharge at 500 Ma to 0.8 volts. The cell was life tested on a three-hour cycle, corresponding to two hours of charging at about 0.55 amperes and one hour of discharging at about 1 amp. Water was added to the cell to make up for losses. Fig. 7A and 7B show strip chart recordings of the cell voltage of cell #113 at periodic intervals through cycle 688. The results obtained show the cell voltages were surprisingly stable during the charge/discharge cycle for more than 500 cycles. Example 2 A cell was constructed as above except that the nickel electrode was of the sintered type and was obtained from a commercial supplier. The 3x3 electrode weighed 12 grams and was 0.71 mm (0.028 inches) thick. The cell was life tested and showed stable cell voltages for more than 200 cycles as shown in Fig. 8 (cell #121). Example 3 A cell was assembled as described above with the exception that the nickel electrode was of the pasted foam type and the separator was a plastic bonded inorganic pigment. Commercially available electroformed nickel foam obtained from Eltek Inc. was pasted with a mixture consisting of a 1% solution of CMC in water which had been added to a dry blended mixture of 10% cobalt monoxide and 90% nickel hydroxide. After drying, the 3x3 inch electrode was pressed to a final thickness of 1.02 mm (0.040 inches). The finished electrode weight was 14 grams. The cell exhibited stable performance for over 1500 cycles as shown in Fig. 9 (cell #144). Example 4 A cell was assembled as described in Example 1 with a hydride electrode comprised of 19% nickel powder (#210 powder obtained from INCO), 1% Teflon®, and 80% hydride alloy. The cell was life tested and showed stable cell voltages for more than 416 cycles as shown in Fig. 10 (cell #134). Example 5 A cell was assembled as described in Example 1 with one layer of a an inorganic composite separator material. The quantity of electrolyte was 8 cc and after the 24 hour soak period all excess free electrolyte was drained from the cell. A layer of porous, wet proofed, graphite paper was placed on the back side of the hydride electrode. The cell was assembled between two acrylic plates with a peripheral rubber gasket. The cell was contained in a sealed compartment and operated in the sealed condition. The cell was life tested and showed stable cell voltages for more than 400 cycles as shown in Fig. 11 (cell #170). A pressure gauge connected to a sensor located in the cell compartment showed no indication of pressures above 1.38 bar (20 psi) during he cycle operation. Example 6 A stack of five vented cells in an arrangement as shown in Fig. 2 was assembled to make a 6 volt battery. The cell construction was similar to Example 1, except the separator was an inorganic composite material. Fig. 12 shows the discharge voltage of cycle 2 and 5. Example 7 Forty grams of commercially available spherical nickel hydroxide was coated with nickel as follows: The nickel hydroxide was soaked in SnCl 2 sensitizer (10 gm SnCl 2 and 40 cc of 12M HCl per liter) solution for about 2.5 minutes. The nickel hydroxide was then filtered and removed from the filter. The wet powder was then soaked in PdCl 2 nucleator (0.25 gm PdCl 2 , 2.5 cc HCl per liter) solution for about 2.5 minutes. The nickel hydroxide was then filtered, washed with about 50 cc of water and removed from the filter. The wet powder was then placed in 2,500 cc of plating solution consisting of water, 105 gm NiSO 4 ·6H 2 O (nickel sulfate), 105 gm NaH 2 PO 2 (sodium hypophosphite), and 210 gm Ha 4 P 2 O7 (sodium pyrophosphate), and adjusted to a Ph of approximately 10 with NH 4 OH. The solution was mildly heated. After plating ceased (the gassing stopped), the powder was filtered and washed with water to neutrality. The wet powder was then vacuum dried. Figs. 13A-13D show scanning electron microscope (SEM) photographs at increasing magnification of particles of the uncoated electrochemically active material and Figs. 14A-14D show SEM photographs of particles of uniformly coated electrochemically active material after being processed according to the method described in this example. The weight gain of nickel in this example was 25%. The bulk resistivity of the coated material was 0.5 to 0.6 ohm-cm when held in compression at 0,986 cm (14.3 psi). Example 8 The same process as in Example 7 was utilized except the metal sulfate was 10% CoSO 4 , 90% NiSO 4 . Example 9 A single cell was fabricated which consisted of one nickel positive electrode and one metal hydride negative electrode assembled in a wafer cell arrangement as shown in Fig. 1. The nickel electrode was prepared by blending a mix consisting of 2 g CoO, 1 g Teflon® powder and 46 g coated nickel hydroxide active material as prepared in Example 7 above. The mix was blended in a high speed blender for two 30 second periods. The mix was then rolled out to a layer of approximately 1.52 mm (0.060 inches) thick, folded and rolled to a 1,52 mm (0.060 inches) thickness in a rotated direction 90 degrees from the original direction. The above folding and thickness reduction in the rotated direction was repeated seven times to a point where the Teflon® powder fibrillated and served as a lace-like network which contained and bonded the other ingredients. The strip was then calendered to a final thickness of 0,69 mm (0.027 inches). A 3x3 inch electrode was cut from the strip. The electrode weighed 11 g. The electrode was then pressed at 345 bar (5000 psi) in a hydraulic press to a thickness of 0.64 mm (0.025 inches) prior to assembly in the cell. The hydride electrode was prepared using substantially the same process as described for the nickel electrode from a mix consisting of 1 g Teflon® powder, 9 g cupric oxide and 40 g of a Mischmetal hydride alloy, such as used in Example 1. The final strip was calendered to a thickness of 0.51 mm (0.020 inches) and a 3x3 inch electrode was cut from the strip. The metal hydride electrode weighed 10 g. One layer of an inorganic separator was assembled between the electrodes. The cell outer faces were constructed from carbon-filled plastic film manufactured by Graphics Inc. A frame border of vinyl plastic was sandwiched between the face layers and heat sealed on three sides. Nickel foil layers having a thickness of 0.05 mm (0.002 inches) were placed on the outer cell faces. The cell assembly was then placed between two rigid Lucite® plates which contained peripheral bolts to hold the assembly together. The cell was filled with 30% KOH, 1% LiOH electrolyte, allowed to soak 24 hours and then subjected to three formation cycles. The cell was life tested on a three hour cycle, 2 hours charge at 0.55 amperes and one hour discharge at 1 ampere. This represented about 66% depth of discharge based on the cell's nominal capacity of 1.5 ampere-hour. Water was periodically added to the cell to make up losses. Fig. 15 shows the voltage at periodic cycles (cycle numbers 108, 206, 290, 395, 535 and 640) for cell #296 when tested on a standard three hour cycle as described in Example 1. The superimposed voltage traces in Fig. 15 show that the voltage remained substantially unchanged for up to 640 cycles. Example 10 A cell (#342) was constructed as in Example 7 with a separator consisting of two layers of a non-woven nylon material. The cell was subjected to three formation cycles and tested at different discharge rates as shown in Fig 16. This results in a Faradaic utilization efficiency for the active material of about 75 to 80% of the maximum theoretical value. Example 11 A cell was constructed as described in Example 7 except the nickel active material was coated as in Example 8. The 3x3 inch electrode weighed 12 g and was 0.028 inches thick. Fig. 17 shows superimposed traces of the cell (#A300) voltage during periodic cycles (#'s 122, 234, 362, 530, 658 and 786) of this cell when tested on a standard three hour cycle as described in Example 1. Example 12 Fig. 18 shows the voltage at periodic cycles (#'s 66, 106, 154, 210, 258 and 330) of a sealed cell #305 constructed as described in example 9 except the cell contained an inorganic composite separator and the cell was life tested on a two hour cycle, 1.5 hours charge at 0.35 amperes and a half hour discharge at 1.0 ampere. The voltage traces remained substantially constant for up to 330 cycles. Example 13 A cell was constructed as above with coated active material of Example 7 except the nickel electrode was of the pasted foam type. Commercially available nickel foam obtained from Eltech Corporation was pasted with a mix consisting of a 1% solution of CMC in water added to a dry blended mix of 2% cobalt monoxide and 98% Ni-coated nickel hydroxide. After drying, the 3x3 inch electrode was pressed to a final thickness off 0.102mm (0.040 inches). The finished electrode weight was 14 g. Fig. 19A shows the cell voltage at periodic cycles (#'s 90, 78, 266, 354, 450 and 554) of this cell when tested on a standard three hour cycle as described in Example 1. The superimposed voltage traces show that the voltage trace remains substantially unchanged for up to 554 cycles. Fig. 19B shows that the minimum voltage for each cycle also remains substantially unchanged for up to 554 cycles. Example 14 A cell was constructed containing one normal 3 ampere-hour laminated mono-polar 7.3 cm x 9.84 cm x 0.13 cm (2 7/8 x 3 7/8 x 0.050 inch) nickel electrode by pressing a layer of plastic bonded electrode strip as prepared in Example 9 on either side of a 0.005 mm (0.002 inch) thick pierced nickel metal foil current collector with tab and nylon separator and to counter hydride electrodes. The nickel electrode delivered 90% Faradaic utilization efficiency on a 3-hour discharge test and 85% Faradaic utilization efficiency on a 20-minute discharge test. A 20-minute discharge test corresponds to 20 minutes of discharging at 9 amperes.

Claims: A method of making an electrode comprising: dry mixing a combination of an electrochemically active material and polytetrafluoroethylene particles in a blending mill to form a mixture; dry rolling said mixture into a thin layer; dry kneading said thin layer by sequentially folding and rolling said thin layer such that said polytetrafluoroethylene particles form an interconnected and fibrous network in which said electrochemically active material is embedded; calendering said folded and rolled thin layer to form a substantially flat porous sheet; and cutting a substantially flat electrode from said porous sheet. The method according to claim 1, wherein said folding and rolling is in a direction about 90 degrees from the folding and rolling of the immediately preceding folding and rolling step. The method according to claim 1 or claim 2, wherein said electrochemically active material comprises metal hydride particles. The method according to any preceding claim, wherein said metal hydride particles have an average particle size of about 20 to about 50 microns. The method according to claim 1 or claim 2, wherein said combination includes cupric oxide particles. The method of according to claim 1 or claim 2, wherein said combination includes silver oxide particles. The method according to claim 1 or claim 2, wherein said electrochemically active material comprises a mixture of nickel hydroxide, graphite and cobalt monoxide. The method according to any preceding claim, wherein said polytetrafluoroethylene particles comprise from about 0.5 to about 5 weight percent of said mixture. The method according to any preceding claim, wherein said polytetrafluoroethylene particles have an average particle size less than about 20 microns.

Inventors: KLEIN MARTIN, KLEIN, MARTIN
Applicants: ELECTRO ENERGY INC, ELECTRO ENERGY, INC.
Application year: 1994
Main classification: H01M10
IPC codes: H01M4, C23C18, H01M10